beneath-apple-dos/D2S1/APX.C.1#064000.txt

.ec]
.bp
.np
.ce
APPENDIX C - GLOSSARY
.sp1
.pn5
.IN20
.PI-20
.PP
ACCESS TIME]>The time required to
locate and read or write data on a
direct access storage device, such as
a diskette drive.
.pp
ADDRESS]>The numeric location of a
piece of data in memory.  Usually
given as a hexadecimal number from
$0000 to $FFFF (65535 decimal).  A
disk address is the location of a
data sector, expressed in terms of its
track and sector numbers.
.pp
ALGORITHM]>A sequence of steps which
may be performed by a program or
other process, which will produce a
given result.
.pp
ALPHANUMERIC]>An alphabetic character
(A-Z) or a numeric digit (0-9).  The
term used to refer to the class of all
characters and digits.
.pp
ANALOG]>As opposed to digital. 
Having a value which is continuous,
such as a voltage or electrical
resistance.
.pp
AND]>The logical process of
determining whether two bits are both
ones.  0 AND 1 results in 0 (false),
1 AND 1 results in 1 (true).
.pp
ARM]>The portion of a disk drive
which suspends the read/write head
over the disk's surface.  The arm can
be moved radially to allow access to
different tracks.
.pp
ASCII]>American Standard Code for
Information Interchange.  A
hexadecimal to character conversion
code assignment, such that the 256
possible values of a single byte may
each represent a alphabetic, numeric,
special, or control character.  ASCII
is used when interfacing to
peripherals, such as keyboards,
printers, or video text displays.
.pp
ASSEMBLY LANGUAGE]>Also known as
MACHINE LANGUAGE.  The native
programming language of the
individual computer.  Assembly
language is oriented to the machine,
and is not humanized, as is BASIC,
PASCAL, or FORTRAN.  An assembler is
used to convert assembly language
statements to an executable program.
.pp
BACKUP]>The process of making a copy
of a program or data against the
possibility of its accidental loss or
destruction.
.pp
BASE]>The number system in use. 
Decimal is base 10, since each digit
represents a power of 10
(1,10,100,...).  Hexadecimal is base
16 (1,16,256,...).  Binary is base 2
(1,2,4,8,...).
.pp
BINARY]>A number system based upon
powers of 2.  Only the digits 0 and 1
are used.  101 in binary, for
example, is 1 units digit, 0 twos,
and 1 fours, or 5 in decimal.
.pp
BIT]>A single binary digit (a 1 or a
0).  A bit is the smallest unit of
storage or information in a computer.
.PP
BIT CELL]>The space on a diskette,
between two clock pulses, which can
hold the value of a single binary 0
or 1 (bit).
.pp
BIT SLIP MARKS]>The epilogue of a
disk field.  Used to double check
that the disk head is still in read
sync and the sector has not been
damaged.
.pp
BOOT/BOOTSTRAP]>The process of
loading a very large program into
memory by loading successively larger
pieces, each of which loads its
successor.  The program loads itself
by "pulling itself up by its
bootstraps".
.pp
BRK]>BREAK.  An assembly langauge instruction
which can be used to force an
interrupt and immediate suspension of
execution of a program.
.pp
BUFFER]>An area of memory used to
temporarily hold data as it is being
transferred to or from a peripheral,
such as a disk drive.
.pp
BUG]>A programming error.  Faulty
operation of a program.
.pp
BYTE]>The smallest unit of
addressable memory in a computer.  A
byte usually consists of 8 bits and
can contain a decimal number ranging
from 0 to 255 or a single
alphanumeric character.
.pp
CARRIAGE RETURN]>A control character
which instructs the printer to end
one line and begin another.  When
printing a carriage return is usually
followed by a line feed.
.pp
CARRY FLAG]>A 6502 processor flag
which indicates that a previous
addition resulted in a carry.  Also
used
as an error indicator by many system
programs.
.PP
CATALOG]>A directory of the files on
a diskette.  See DIRECTORY.
.pp
CHAIN]>A linked list of data
elements.  Data is chained if its
elements need not be contiguous in
storage and
each element can be found from its
predecessor via an address pointer.
.pp
CHECKSUM/CRC]>A method for verifying
that data has not been damaged.  When
data is written, the sum of all its
constituent bytes is stored with it. 
If, when the data is later read, its
sum no longer matches the checksum,
it has been damaged.
.pp
CLOBBERED]>Damaged or destroyed.  A
clobbered sector is one which has
been overwritten such that it is
unrecoverable.  
.pp
CODE]>Executable instructions to the
computer, usually in machine
language.
.pp
COLDSTART]>A restart of a program
which reinitializes all of its
parameters, usually erasing any work
which was in progress at the time of
the restart.  A DOS coldstart erases
the BASIC program in memory.
.pp
CONTIGUOUS]>Physically next to.  Two
bytes are contiguous if they are
adjoining each other in memory or on
the disk.
.pp
CONTROL BLOCK]>A collection of data
which is used by the operating system
to manage resources.  Examples of a
control block used by DOS are the
file buffers.
.pp
CONTROL CHARACTER]>A special ASCII
code which is used to perform a
unique function on a peripheral, but
does not generate a printable
character.
Carriage return, line feed, form
feed, and bell are all control
characters.
.pp
CONTROLLER CARD]>A hardware circuit
board which is plugged into an APPLE
connector which allows communication
with a peripheral device, such as a
disk or printer.  A controller card
usually contains a small driver
program in ROM.
.PP
CSWL]>A vector in zero-page through
which output data is passed for
display on the CRT or for printing.
.pp
CYCLE]>The smallest unit of time
within the central processor of the
computer.  Each machine language
instruction requires two or more
cycles to complete.  One cycle (on
the APPLE) is one micro-second or one
millionth of a second.
.pp
DATA]>Units of information.
.pp
DATA SECTOR BUFFER]>On the APPLE, a
256 byte buffer used by DOS to hold
the image of any given sector on the
diskette.  As information is read
from the file, data is extracted from
the data sector buffer until it is
exhausted, at which time it is
refilled with the next sector image.
.pp
DATA TYPE]>The type of information
stored in a byte.  A byte might
contain a printable ASCII character, binary
numeric data, or a machine language
instruction.
.pp
DCT]>Device Characteristics Table. 
Used as an input parameter table to
Read/Write Track/Sector (RWTS) to
describe the hardware characteristics
of the diskette drive.
.pp
DECIMAL]>A number system based upon
powers of 10.  Digits range from 0 to
9.
.pp
DEFERRED COMMANDS]>DOS commands which
may (or must) be invoked from within
an executing BASIC program.  OPEN,
READ, WRITE, and CLOSE are all
examples of deferred commands.
.pp
DIGITAL]>As opposed to analog. 
Discrete values as opposed to
continuous ones.  Only digital values
may be stored in a computer.  Analog
measurements from the real world,
such as a voltage or the level of
light outside, must be converted into
a numerical value which, of
necessity, must be "rounded off" to a
discrete value.
.pp
DIRECT ACCESS]>Peripheral storage
allowing rapid access of any piece of
data, regardless of its placement on
the medium.  Magnetic tape is
generally not considered direct
access, since the entire tape must be
read to locate the last byte.  A
diskette is direct access, since the
arm may be rapidly moved to any
track and sector.
.pp
DIRECTORY]>A catalog of all files
stored on a diskette.  The directory
must contain each file's name and its
location on the disk as well as other
information regarding the type of
data stored there.
.pp
DISK INITIALIZATION]>The process
which places track formatting
information, including sectors and
gaps, on a blank diskette. 
During disk initialization, DOS also
places a VTOC and directory on the
newly formatted disk, as well as
saving the HELLO program.
.pp
DISPLACEMENT]>The distance from the
beginning of a block of data to a
particular byte or field. 
Displacements are usually given
beginning with 0, for the first byte,
1 for the second, etc.  Also known as
an offset.
.pp
DRIVER]>A program which provides an
input stream to another program or an
output device.  A printer driver
accepts input from a user program in
the form of lines to be printed, and
sends them to the printer.
.pp
DUMP]>An unformatted or partially
formatted listing of the contents of
memory or a diskette in hexadecimal. 
Used for diagnostic purposes.
.pp
ENCODE]>To translate data from one
form to another for any of a number
of reasons.  In DOS 3.3,
Data is encoded from 8
bit bytes to 6 bit bytes for storage
on a DISK II.
.pp
ENTRY POINT (EPA)]>The entry point
address is the location within a
program where execution is to start. 
This is not necessarily the same as
the load point (or lowest memory
address in the program).
.pp
EOF]>End Of File.  This mark signals
the end of a data file.  $00 for
APPLE DOS text files.
.pp
EPILOGUE]>The last three bytes of a
field on a track.  These unique bytes
are used to insure the integrity of
the data which preceeds them.
.pp
EXCLUSIVE OR]>A logical operation
which compares two bits to determine
if they are different.  1 EOR 0
results in 1.  1 EOR 1 results in 0.
.pp
FIELD]>A group of contiguous bytes
forming a single piece of data, such
as a person's name, his age, or his
social security number.  In disk
formatting, a group of data bytes
surrounded by gaps.
.pp
FILE]>A named collection of data on a
diskette or other mass storage
medium.  Files can contain data or
programs.
.pp
FILE BUFFERS]>In APPLE DOS, a
collection of buffers used to manage
one open file.  Included are a data
sector buffer, a Track/Sector List
sector buffer, a file manager
workarea buffer, the name of the
file, and pointers.  The DOS command,
MAXFILES 3, causes 3 of these file
buffers to be allocated.
.pp
FILE DESCRIPTOR]>A single entry in a
diskette directory which describes
one file.  Included are the name of
the file, its data type, its length,
and its location on the diskette.
.pp
FILE MANAGER]>That portion of DOS
which manages files.  The file
manager handles such general
operations as OPEN, CLOSE, READ,
WRITE, POSITION, RENAME, DELETE, etc.
.pp
FILE TYPE]>The type of data held by a
file.  Valid DOS file types are
Binary, Applesoft, Integer-BASIC,
Text, Relocatable, S, A, and B.
.pp
FIRMWARE]>A middle ground between
hardware and software.  Usually used
to describe micro-code or programs
which have been stored in read-only
memory.
.pp
GAPS]>The spaces between fields of
data on a diskette.  Gaps on an APPLE
diskette contain self-sync bytes.
.pp
HARD ERROR]>An unrecoverable
Input/Output error.  The data stored
in the disk sector can never be
successfully read again.
.pp
HARDWARE]>Physical computer
equipment, as opposed to programs
which run on the equipment.
A disk drive is an example of a
hardware component.
.pp
HEAD]>The read/write head on a
diskette drive.  A magnetic pickup,
similar in nature to the head on a
stereo tapedeck, which rests on the
spinning surface of the diskette.
.pp
HEXADECIMAL/HEX]>A numeric system
based on powers of 16.  Valid hex
digits range from 0 to 9 and A to F,
where A is 10, B is 11, ... , and F
is 15.  B30 is 11 256's, 3 16's, and
0 1's, or 2864 in decimal.
Two hexadecimal digits can be used to
represent the contents of one byte.
Hexadecimal is used with computers
because it easily converts with
binary.
.pp
HIGH MEMORY]>Those memory locations
which have high address values. 
$FFFF is the highest memory location.
Also called the "top" of memory.
.pp
HIMEM]>APPLE's zero-page address
which identifies the first byte past
the available memory which can be
used to store BASIC programs and
their variables.
.pp
IMMEDIATE COMMAND]>A DOS command
which may be entered at any time,
especially when DOS is waiting for a
command from the keyboard.  Deferred
commands are the opposite of
immediate commands.
.pp
INDEX]>A displacement into a table or
block of storage.
.pp
INSTRUCTION]>A single step to be
performed in an assembly language or
machine language program. 
Instructions perform such operations
as addition, subtraction, store, or
load.
.pp
INTEGER]>As opposed to floating
point.  A "whole" number with no
fraction associated with it.
.pp
INTERCEPT]>A program which logically
places itself in the execution path
of another program, or pair of
programs.  A video intercept
is used to re-direct program output
from the screen to a printer,
for example.
.pp
INTERLEAVE]>The practice of selecting
the order of sectors on a diskette
track to minimize access time due to
rotational delay.  Also called
"skewing" or interlacing.
.pp
INTERRUPT]>A hardware signal which
causes the computer to halt execution
of a program and enter a special
handler routine.  Interrupts are used
to service real-time clock
time-outs, BRK instructions, and
RESET.
.pp
IOB]>Input/Output Block.  A
collection of parameter data, passed
to Read/Write Track/Sector,
describing the operation to be
performed.
.pp
I/O ERROR]>Input/Output Error.
An error which occurs
during transmission of data to or
from a peripheral device, such as a
disk or cassette tape.
.pp
JMP]>A 6502 assembly langauge
instruction which causes the computer
to begin executing instructions at a
different location in memory. 
Similar to a GOTO statement in BASIC.
.pp
JSR]>A 6502 assembly langauge
instruction which causes the computer
to "call" a subroutine.  Similar to a
CALL statement in BASIC.
.pp
K]>A unit of measurement, usually
applied to bytes.  1 K bytes is
equivalent to 1024 bytes.
.PP
KSWL]>A vector in zero-page through
which input data is passed for
from the keyboard or a remote
terminal.
.pp
LABEL]>A name associated with a
location in a program or in memory. 
Labels are used in assembly langauge
much like statement numbers are used
in BASIC.
.PP
LATCH]>A component into which the
Input/Output
hardware can store a byte value,
which will hold that value until the
central processor has time to read
it (or vice versa).
.pp
LINK]>An address pointer in an
element of a linked chain of data or
buffers.
.pp
LIST]>A one dimensional sequential
array of data items.
.pp
LOAD POINT (LP)]>The lowest address
of a loaded assembly language
program -- the first byte loaded. 
Not necessarily the same as the entry
point address (EPA).
.pp
LOGICAL]>A form of arithmetic which
operates with binary "truth" or
"false", 1 or 0.  AND, OR, NAND, NOR,
and EXCLUSIVE OR are all logical
operations.
.pp
LOOP]>A programming construction in
which a
group of instructions or statements
are repeatedly executed.
.pp
LOW MEMORY]>The memory locations with
the lowest addresses.  $0000 is the
lowest memory location.  Also called
the "bottom" of memory.
.pp
LOMEM]>APPLE's zero-page address
which identifies the first byte of
the available memory which can be
used to store BASIC programs and
their variables.
.pp
LSB/LO ORDER]>Least Significant Bit
or Least Significant Byte.  The 1's
bit in a byte or the second pair of
hexadecimal digits forming an
address.  In the address $8030, $30
is the LO order part of the address. 
.pp
MASTER DISK]>A DOS diskette which
will boot in an APPLE II of any size
memory and take full advantage of it.
.pp
MICROSECOND]>A millionth of a
second.  Equivalent to one cycle of
the APPLE II central processor.
Also written as "Usec".
.pp
MONITOR]>A machine language program
which always resides in the computer
and which is the first to receive
control when the machine is powered
up.  The APPLE monitor resides in ROM
and allows examination and
modification of memory at a byte
level.
.pp
MSB/HI ORDER]>Most Significant Bit or
Most Significant Byte.  The 128's bit
of a byte (the left-most) or the
first pair of hexadecimal digits in
an address.  In the byte value $83,
the MSB is on (is a 1).
.pp
NULL]>Empty, having no length or
value.  A null string is one which
contains no characters.  The null
control character ($00) produces no
effect on a printer (also called an
idle).
.pp
NIBBLE/NYBBLE]>A portion of a byte,
usually 4 bits and represented by a
single hexadecimal digit.  $FE
contains two nibbles, $F and $E.
.pp
OBJECT CODE]>A machine language
program in binary form, ready to
execute.  Object code is the output
of an assembler.
.pp
OBJECT MODULE]>A complete machine
language program in object code form,
stored as a file on a diskette.
.pp
OFFSET]>The distance from the
beginning of a block of data to a
particular byte or field. 
Offsets are usually given
beginning with 0, for the first byte,
1 for the second, etc.  Also known as
a displacement.
.pp
OPCODE]>Operation Code.  The three
letter mnemonic representing a single
assembly langauge instruction.  JMP
is the opcode for the jump
instruction.
.pp
OPERATING SYSTEM]>A machine language
program which manages the memory and
peripherals automatically,
simplifying the job of the
applications programmer.
.pp
OR]>The logical operation comparing
two bits to determine if either of
them are 1.  1 OR 1 results in 1
(true), 1
OR 0 results in 1, 0 OR 0 results in
0 (false).
.pp
OVERHEAD]>The space required by the
system, either in memory or on the
disk, to manage either.  The disk
directory and VTOC are part of a
diskette's overhead.
.br
.nx appendix c.2
\x00